Selenium contact electrode



March 1940. E. D. wLsoN V 2,193,610

y SELENIUM CONTACT ELECTRODE Filed Feb. 17, 1938/l/l//I/l/l///I/I/I//I/l/ m (wwseleniun ATTORN I Patented Mar. 12, 1940UNITED STATES SELENIUM CONTACT ELECTRODE Earl D. Wilson, Wilkinsburg,Pa., assignor to Westinglouse Electric & Manufacturing Company, EastPittsburgh, Pa., a corporation oi' Pennsylvania Application February 17,1938, Serial No. 190395 1 Claim.

My invention relates to dry rectiflers and especially to the seleniumdry rectifier.

An object of the invention is to provide a very efiicient seleniumrectifier.

Another object of the invention is to provide a very intimate contact byelectrodes'to the rectifying layer of selenium.

Another object is to provide a method of reducing the reverse current inselenium rectifiers.

Other objects and advantages of the invention will be apparent from thefollowing description and drawing, in which the figure is across-section through a preferred type of rectifier embodying theinvention.

u In forming the rectifier, I preferably take as an electrode a materialof the iron group, and particularly a plate of cold rolled steel such asthat illustrated in cross-section in the drawing. One surface of thiselectrode is roughened to a go mat surface by any convenient processsuch as sand blasting or acid etching. Upon this surface I coat therectifying layer which may be any one or combination of the selenium orsulphur group. I prefer, however, to coat the roughened surface of theplate with amorphous selenium at a temperature between 220C. and 250C.and to apply the selenium to a thickness of several mils and to cool itquickly. This selenium is preferably of a commercially available qualityhaving the highest order of electrical conductance when in the metallicform.

The coated steel plate is then preferably heated to approximately 130 C.and is pressed against a smooth flat surface with sufficient force toreduce the plastic selenium to a smooth uniform layer preferably abouttwo mils thick. This smooth flat surface may be conveniently provided bythe use of a phenol condensation product, H especially that sold underthe trade name of "MicartaJ' The plate is then put preferably into" anoven at room temperature and heated gradually to approximately 180 to190 C. within a period of two hours, after which it is annealed at theacquired temperature for several hours, I' preferably at least six. Inpractice, this can be Conveniently done overnight. The oven is thenpreferably gradually cooled to room temperature before the plate isremoved, the time taken being preferably that of three hours.

In order to provide an intimate contact with the other electrodethereon, I prefer to `spray the material of the other electrode onto theselenium surface. I have discovered that cadmium in the sprayedelectrode contributes towards the formation of a rectifying layerbetween the selenium and the electrode coating thereon. A preferredcadmium alloy is that of the eutectic of cadmium and tin which isapproximately '75% tin and 25% cadmium with a melting point about 165 C.The percentage of cadmium may be varied from 100% or all cadmium to thatof a small percentage, for example 0.1% with the tin.

A typical element made as specified will deliver about amperes persquare inch in the forward direction at 3 volts, and about .03 amperefor the same voltage in the reverse direction.

I have also discovered that the reverse current can be still furtherreduced by applying suificient alternating-current voltage to theelement to bring the temperature momentarily to approximately 150 to175`C. Repeated applicatons of this treatment may reduce the reversecurrent by a factor of 5 or with only a relatively slight decrease inforward conductance.

Many modifications may be made in the form and arrangement of the partsdisclosed in the rectifier, and also various modifications may be madein the various steps and temperatures in forming the selenium rectifier.Accordingly, I desire only such limitations to be imposed upon thefollowing claim as is necessitated by the prior art.

I claim as my invention:

The method of reducing the back current in a selenium rectifier whichcomprises applying sufl'icient alternating-current voltage thereto tobring the temperature momentarily to approximately 150 to 175 C.

EARL D. WIISON.

